The present invention relates to energy conversion apparatus and especially to Stirling engines which are structured to operate with substantially increased efficiency to enable them to be widely applicable for general commercial purposes and further to internal combustion engines which are structured to operate with increased efficiency and reduced pollution.
The Stirling engine was first developed early in the Nineteenth Century. It is a closed cycle air engine that employs a regenerator to reuse heat that would otherwise be wasted. Early Stirling engines were characterized with metallurgical problems, especially since the structural materials available at the time were not well suited to the continuously high temperatures at which hot parts of the engine were operated. Competing steam engines operated at lower temperatures and thus had no need for materials capable of operating at the high operating temperatures of Stirling engines.
The early Stirling engines were used for a wide variety of purposes, but limited power generating capability and limited reliability restricted the Stirling engine in its commercial applications and usage. On the other hand, inducements did exist for use of Stirling engines because of their quiet operation, because of their capability for using a wide variety of fuels, and because of their greater appropriateness for certain uses for which steam engines were unsuitable.
With the development of the electric motor and the gasoline engine as load drives, usage of the Stirling engine declined since the new load drives provided greater power with reduced engine size and, in general, operated with greater efficiency. By the early part of the Twentieth Century, Stirling engines existed essentially only as models of the past.
Nonetheless, around the middle of the Twentieth Century, development activity was continued on the Stirling engine concept, especially for communications and vehicular applications. Development incentives generally arose from the availability of stainless steel and other materials capable of operating at high temperatures, and from the promise of success based on a gap that then existed between theoretical Stirling cycle efficiency and the actual efficiency of existing Stirling engines.
As a result of early development efforts primarily for power generation in communication applications, improvements were made in ratios of engine power to engine weight and engine power to engine size. Stirling engines could be made with operating speeds in excess of 3000 revolutions per minute (rpm) and with power generating capabilities up to and in excess of 25 horsepower.
Smaller Stirling engines tended to employ Stirling's original piston-displacer design, whereas the larger Stirling engines tended to employ a double acting design called a Rinia engine. Both designs employed pressurized air as a closed cycle working gas. However, sealing problems were experienced with the Rinia engine design.
A twin crankshaft drive called a rhombic drive then became available to reduce concentric push-rod sealing problems and to enable use of the original Stirling piston-displacer arrangement without a large pressurized crankcase. As a result, participation of the automotive industry in Stirling engine development increased significantly. Higher powered Stirling engines were developed with much higher working gas pressures and with energy conversion efficiencies as good as or better than those of the gasoline and diesel internal combustion engines. Small Stirling engines were developed with operating speeds as high as 5000 rpm.
More recently, further interest in Stirling engine development arose from concerns about automotive emissions and increasing oil prices caused by monopolistic oil pricing practices. The Stirling engine is essentially an environmentally clean engine, and, as previously noted, has a capability for using a wide variety of fuels which could lead to substantially reduced use of petroleum products in favor of cost-effective, non-polluting alternative fuels.
The rhombic drive, which had previously appeared to be a promising engine development, eventually became undesirable because of its manufacturing difficulty and its high manufacturing cost. Consequently, development efforts, especially in the automotive industry, were generally redirected to the basic Rider design without the rhombic drive.
Research and development efforts have continued on Stirling engines, but the availability of acceptable, relatively low cost, and well developed internal combustion or other engines (such as those in the automotive industry) has limited the scope and depth of such efforts. Currently, Stirling engines are still burdened by limited efficiency and excessive size and weight relative to produced power in various applications, such as, automobiles, powerplants, boats, lawn mowers, etc.
With one exception, there are no developed Stirling engines available for high power applications, even though the Stirling engine is essentially environmentally clean and has the capability of using a wide variety of fuels. Sweden has developed a Stirling engine to drive a 75 kW generator. Two of these engines are used in the Gotland class (Type A-19) submarine. The next generation "Submarine 2000" is expected to use a Stirling engine for propulsion.
Classically, Stirling engines have thus been used to provide direct mechanical power to operate stationary machinery, to propel tractors, trucks and automobiles, to generate electricity for lighting, heating, communication equipment, computers, and electrochemical processes, and to power electric generators for both stationary and mobile applications. With new emphasis on the conservation of resources, it is becoming increasingly important to develop engines that are more efficient and that are capable of using a variety of fuels. The development of non-polluting engines is also important so that engine operation introduces a minimum of pollution into the atmosphere and generates a minimum of hazardous waste products. Additionally, it is becoming increasingly important to develop engines that produce power without excessive noise levels and without a requirement for noise suppression apparatus that otherwise reduces efficiency.
Engine maintenance is also an important consideration. Engines that require frequent lubrication changes increase operating costs in two ways, direct cost of the lubricants and the cost of disposing of the lubricating fluids in a way that is environmentally safe.
The Stirling engine meets all of the desirable requirements described above, except one, i.e., efficiency. With increased efficiency, the Stirling engine may ultimately be capable of being used in a wide variety of applications, including motor vehicle engines, large station power generators, space and other remote power generators, etc., and such applications can be environmentally clean and quiet with use of any of a wide variety of fuels. Further, the size and weight of Stirling engines employed for such uses can be reduced if engine efficiency is increased.
Although there has been a reemphasis in the last twenty years on Stirling engine improvements with newer materials and higher operating temperatures and pressures that have reduced size and weight, little has been done to reshape the actual Stirling thermodynamic cycle so that it more nearly approaches the desired theoretical efficiency to enable substantial increased engine efficiency. In any case, commercial use of Stirling engines has been highly restricted or non-existent in various applications including power plants, vehicles, lawn mowers, and other apparatus, even in the face of the great Stirling engine attraction of full flexibility, and quiet and non-polluting operations.
Thus, a major need exists for development of the application of Stirling engines in various systems. Further significant improvement in the energy conversion efficiency of Stirling engines is a key to such system developments.
As previously indicated, internal combustion engines have been highly developed and widely used as a result of extensive industrial research and development efforts especially in the production of automobiles and other vehicles. Accordingly, it is also desirable to provide internal combustion engines which operate with better efficiency and reduced atmospheric pollution, even though internal combustion engines lack the fuel flexibility and the quiet, non-polluting operation of Stirling engines.
It is therefore desirable that improvements, having special utility in improving Stirling engine applications, also be adopted insofar as possible to provide improvement in internal combustion engine applications.